1. Technical Field
The present invention relates to an ink jet recording method.
2. Related Art
The ink jet recording method is a technique for printing performed by ejecting droplets of an ink onto a recording medium, such as a paper sheet, from an ink jet head. The ink jet recording method is being innovatively developed and increasingly applied to high-resolution image recording (printing), which has been performed by photo printing and offset printing. For the ink jet recording method, it is desirable that ink ejection nozzles of ink jet heads be prevented from being clogged.
One of the ink jet recording methods has been known in which a line head including ink ejection nozzles linearly aligned ejects droplets of an ink composition (hereinafter may be referred to as ink droplets) onto a recording paper being transported at a speed according to the ejection speed and volume of ink droplets. In order to increase the resolution of the image printed by this recording method, an arrangement of ink jet heads each including nozzles aligned in a straight line is proposed. In this arrangement, the ink jet heads are disposed in the direction along the width of the recording medium in a staggered manner across the entire width of the recording medium with no spaces so that the pitch of the nozzles is reduced to reduce the dot pitch.
In this instance, solid printing can be performed by overlapping ink jet heads adjacent to each other in the direction in which the recording medium is transported (hereinafter referred to as the transport direction). However, if the volume of the ink droplet ejected from the ink ejection nozzles in the overlap portion of each ink jet head overlapping with the adjacent ink jet head is set to be equal to the volume of the ink droplet ejected from the nozzles in the non-overlap portion or middle portion of the ink jet head, the overlap portions of the two ink jet heads eject ink droplets to the same position on the recording medium. Consequently, a striped pattern of nonuniform density is formed in the image. Such nonuniformity in density can be prevented by setting the ejection volume so that the ink ejection nozzles in the middle portion eject a larger volume of ink droplet than the ink ejection nozzles in the overlap portion. For example, JP-A-2007-185904 discloses an ink jet image printing system that perform printing on an image printing medium using a line head having overlap portions in which a predetermined number of nozzles of each head chip are aligned with nozzles of the adjacent head chip. The ink jet image printing system includes a density detector that detects the density of the image formed, an image memory module in which image data is stored, and a controller that controls the amount of ink to be ejected from the overlap portion of the head chips according to the difference in image density between an image formed by the overlap portion and an image formed by the middle portion of the head chip, obtained from the image data read out of the image memory module.
The above cited patent document describes that if the ink ejection nozzles of the overlap portion are slightly displaced along the line of the nozzle alignment by the displacement of an ink jet head to increase or reduce the nozzle-to-nozzle distance between the heads relative to the nozzle-to-nozzle distance in a head, the image printed on the recording medium exhibits nonuniformity in density or undesired white stripes at the portion corresponding to the joint of the heads. According to this document, the above system can solve such an issue.
However, the present inventors have conducted detailed research on the system disclosed in the above-cited document and found that even this system cannot sufficiently solve the issue of the striped pattern of nonuniform density in printed images.